1. Field of the Invention
This invention relates to a finder used in a camera or the like, and particularly to a changeable magnification inverted Galilean finder.
2. Description of the Prior Art
In a focal length change-over type camera, it is desired that with a change of the angle of view of the photo-taking lens, a photographing range display be changed in the finder. In a camera containing an inverted Galilean finder therein, changing the size of the field frame and changing the magnification of the finder are two known techniques for changing the photographing range display. For changing the size of the field frame, there is known, for example, a method of providing an illuminating window type bright frame and moving the bright frame to thereby vary the apparent size of the frame, but this method has a disadvantage that if the angle of view becomes narrow, the apparent field of the field frame becomes small and it becomes difficult to look through the field frame. In contrast, in a system for changing the magnification of the finder, it is possible to make the apparent field of the photographing range constant so that a finder observation more approximate to the actual photographing picture plane becomes possible. This system is thus superior to changing the size of the field frame.
As the method of changing the magnification, the focal length of a negative lens as an objective lens may be varied and an afocal state may be kept between the negative lens and an eyepiece of positive refractive power. More specifically, there is known a method in which, as shown in FIGS. 1A and 1B of the accompanying drawings, a negative lens L2 is replaced with a negative lens L1 having a different focal length, or a method in which, as shown in FIGS. 2A and 2B of the accompanying drawings, a negative lens L4 is removed from the optical path and a negative lens L1 is moved toward the observation side to thereby effect the change of the magnification to a high magnification. FIGS. 1A and 2A show the low magnification state, i.e., the wide angle side, and FIGS. 1B and 2B show the high magnification state, i.e., the telephoto side. These methods are methods of changing the magnification by positioning a negative lens onto or of the optic axis, but these methods have a disadvantage that the amounts of displacement S1 and S2 of the negative lens in the direction of the optic axis during a change between high magnification and great, so that magnification are great and it is difficult to provide sufficient space on the eyepiece side and therefore, the construction of the finder is liable to become bulky. In a finder for a camera, it is desirable to set the full length of the finder to a value equal to or shorter than the thickness of the body of the camera and in practice, the full length of the finder is desirably of the order of 30-40 mm. Particularly, in the latest finders, it is desired that numerous types of information such as the field frame, the range finding frame, the photographing distance display and the propriety of exposure amount be displayed at the same time and therefore, it has become necessary to add space-consuming members such as an Albada system and a half-mirror for the illuminating window type bright frame. However, where the amount of displacement of the negative lens for magnification change on the optic axis is great, the space in which optical members for various information displays are disposed becomes smaller and thus, design limitations have become more severe and sufficient information displays have been difficult.